1. Technical Field of the Invention
The present invention disclosed herein relates to semiconductor memory devices and more particularly, to nonvolatile semiconductor memory devices.
2. Description of the Related Art
Flash memory devices are kinds of nonvolatile memories capable of keeping data stored therein even without power supply. With a rapid increase of mobile apparatuses such as cellular phones, personal digital assistants (PDA), digital cameras, portable gaming consoles, and MP3, flash memory devices are widely employed for executable code storage, as well as for data storage. Flash memory devices may be also utilized in home applications such as high-definition televisions, digital versatile disks (DVDs), routers, and global positioning systems (GPS). The development of technologies for fabricating smaller but high circuit-density semiconductor chips for use in the above electronic devices also increased the need for higher density memories. A variety of methods for enhancing the integration density of memories have been proposed in the art. One of these methods is to make a memory device in a three-dimensional array structure (hereinafter, referred to ‘3-dimensional memory device’ or ‘vertical’ array memory structure). Technical details for conventional 3-dimensional memory devices are disclosed in U.S. Pat. No. 5,835,396 (issued on Dec. 7, 1998) entitled “THREE-DIMENSIONAL READ-ONLY MEMORY”; U.S. Pat. No. 6,034,882 (issued on Mar. 7, 2000) entitled “VERTICALLY STACKED FIELD PROGRAMMABLE NONVOLATILE MEMORY AND METHOD OF FABRICATION”; and U.S. Pat. No. 7,002,825 (issued on Feb. 21, 2006) entitled “WORD LINE ARRANGEMENT HAVING SEGMENTED WORD LINES”, and U.S. Patent Publication No. 2007-0252201 entitled “NONVOLATLE SEMICONDUCTOR MEMORY DEVICE AND MANUFACTURING METHOD THEREOF”, all of which are incorporated by reference herein.